Post-embryonic photoreceptor development and dark/light adaptation in the stick insect Carausius morosus (Phasmida, Phasmatidae)

The aims of this paper have been (a) to document postembryonic eye growth in the common laboratory stick insect Carausius morosus and (b) to examine whether the capacity of the eye to adapt to changing light intensities varied with age. We found that number of facets, corneal thickness, ommatidial diameter, widths of cone and retinal layers, and rhabdom volume increased linearly. Interommatidial angles, pigment grain sizes, and microvillar diameters, however, remained approximately the same. On the basis of the morphometric data we calculated that sensitivity in the adult stick insect eye was at least tenfold that of the eye of first instar nymphs and could show that adults would be able to perceive a similar amount of detail at considerably dimmer ambient light than the smaller individuals. In terms of resolving power this means that light- and dark-adapted first instar stick insects possess acceptance angles of 5.3degrees and 8degrees, respectively and that the corresponding figures for the adult eye are 4.7degrees and 7.3degrees. The bigger (and more light-sensitive) eye of fully grown C morosus makes protection against radiation damage a more serious issue in the adult individual than the small first instar. The findings explain why smaller (=younger) stick insects are less nocturnal than mature, fully-grown individuals. It is, thus, not surprising to see that dark/light adaptational photomechanical changes affecting pigment position and rhabdom widths, are more pronounced in the eyes of the adults.